Polyimide compounds are widely used in films, wire coating materials and electronic and electric materials. It is impossible to rearrange polyimide compounds, due to their molecular structure and because said compounds have strong chemical bonds and resonance stability.
Prior art polyimide compounds are used in the form of a solvent-insoluble film, which is prepared by polymerizing an aromatic tetracarboxylic dianhydride such as pyromellitic anhydride, with an aromatic diamine in a polar solvent to give an aromatic polyamic acid, applying the resultant compound onto a substrate and heating it.
Prior art aromatic-based siloxane modified polyimides are classified into two types: one is the siloxane-based polyimide that is used in liquid crystal alignment layer (U.S. Pat. No. 4,405,208; Japanese Laid Open Patent Publication No. (Sho) 60-91329), adhesives (U.S. Pat. No. 4,011,279), composite materials (U.S. Pat. Nos. 3,740,305, and 4,395,527) and the like, and the other is the siloxane-based polyimide that is used as the protective coatings for semiconductors (U.S. Pat. Nos. 3,615,913, and 4,030,948).
However, the high glass transition temperature and the absence of a suitable solvent renders the prior art polyimide compounds difficult in processing. In addition, since the precursor polyamic acid is sensitive to moisture, heat and temperature, when it stands at room temperature an undesirable whitening phenomenon is caused wherein the viscosity of polyamic acid solution decreases and the solution is partially ring-closed and becomes insoluble. Furthermore, the prior art preparation of the polyimide compound is disadvantageous in view of the energy saving and protection of semiconductor components, since polyamic acid should be heated at a high temperature of above 300.degree. C. for long periods, so that the polyamic acid coated on the substrate is imidized. There are also limitations in its use for semiconductor protective coatings, liquid crystal alignment layer, and the like, since the prior art polyimide adheres poorly to substrates such as silicon wafer and has low clearness.
In order to solve such drawbacks, aromatic radicals are introduced into molecular chains (Japanese Laid Open Patent Publication No. (Sho) 58-17418) or sulfone-based monomers are incorporated into molecular chains (U.S. Pat. No. 4,874,834) in order to increase the effective volume of the polymer itself, whereby the transparency is improved, or the heat resistant property is improved by using fluoro-substituted monomers (U.S. Pat. No. 4,781,439).
Japanese Laid Open Patent Publication Nos. (Sho) 63-205322 and 60-166325 and U.S. Pat. No. 4,668,755 disclose a method for preparing siloxane-modified polyimide using acid dianhydride, aromatic diamine and siloxane diamine.
When the polyimide prepared by the above methods is used as coatings for a semiconductor and other electronic devices, the adhesion to the substrate, such as a silicon wafer, is excellent, due to the introduction of silicon radicals. However, if the heat resistance is increased, then mechanical properties such as tensile strength and elongation are decreased. In contrast, if the mechanical properties are increased, then the heat resistance is decreased.
The present inventors have investigated the prior art problems in order to solve them. As a result, it is found that the synthesis of siloxane modified polyimide resin using specific diamine components provides the key to solve the prior problems. The present invention is based on such a discovery.
Polyimide resin prepared by the present invention provides a high heat resistant property and processability, good adhesion to the silicon wafer substrate due to the presence of silicone in the molecular structure, and high transparency owing to the good dissolving property of the polyamic acid in polar solvents. Thus, the polyimide resin prepared by the present invention is suitable for use as protective coatings for semiconductors and liquid crystal alignment layer.